Electromeric Effect

Electromeric Effect

In presence of an attacking reagent, there is complete transfer of $\pi -$electrons from one atom to other to produce temporary polarity on atoms joined by multiple bonds, it is called Electromeric

This effect is temporary and takes place only in the presence of a reagent. As soon as the reagent is removed, the molecule reverts back to its original position. Electromeric effect is of two types, i.e. $+Eeffectand-Eeffect.$

Positive Electromeric effect (+E effect):

When $\pi$electrons transfer takes place C to C (as in alkenes, alkynes etc.), it is called positive electromeric effect (denoted by +E)

$C{H}_2=C{H}_2\stackrel{+}{C}{H}_2-\stackrel{-}{C}{H}_2$

$C{H}_3-CH=C{H}_2\stackrel{+}{C{H}_3-C}H-\stackrel{-}{C}{H}_2$

For example, addition of acids to alkenes

In this, there is also (+I) effect of $-C{H}_3$ group which causes $\pi electrontransferfrom{C}_2$ to C₁. What do you think in the following case:

(+I) effect of $C{H}_{3}C{H}_2-$is larger than that of $C{H}_3^{-},hence\pi$electron transfer is from $C_{3}to{C}_2.$

Negative Electromeric effect:

When $\pi -$ electrons transfer takes places to more electronegative atom (O, N, S) joined by multiple bonds, it is called Negative Electromeric effect (denoted by -E).

for example, the addition of cyanide ion to the carbonyl group.